To examine the role of single allele lipoprotein lipase (LpL) structural defects in lipoprotein metabolism, and explain the possible reasons for altered clinical expression in heterozygous LpL deficiency, we studied 14 patients with heterozygous LpL deficiency and 3 homozygotes (LpL defects in both alleles). Subjects consumed an average American diet for four weeks to achieve dietary stabilization. ApoB-100 (LDL, IDL, and VLDL), and apoB48 (chylomicrons/remnants), production and catabolic rates were measured on a metabolic ward, during a 15-hour infusion of deuterated leucine in the continuously fed state, as well as a bolus infusion of 13C-leucine.